Extrusion dies are used to apply coatings to substrates to form various products. Tubular products are generally formed by coating extrusion material on wire-like substrates, and film products are formed by coating extrusion material on sheet-like substrates.
FIG. 1 is schematic diagram of an extrusion system 100. The system 100 generally includes an extrusion material 102, a substrate 104 that is to be coated with the extrusion material 102, an extrusion die 106, a cooling bath 108, and a substrate puller 110. The extrusion material 102 is typically a polymer (i.e., plastic) that is heated to a molten form. The extrusion material 102 is typically driven by a screw device that provides a near continuous volumetric flow rate of the extrusion material 102 to the extrusion die 106.
The substrate 104 (e.g., wire-like or sheet-like) is pulled through the extrusion die 106 by the substrate puller 110 at a desired rate while the extrusion material 102 is simultaneously fed into the extrusion die 106. Both the substrate 104 and the extrusion material 102 travel in a longitudinal direction (indicated by arrow 112) through the extrusion die 106.
The extrusion die 106 typically includes a die component having a fixed channel through which the substrate 104 and the extrusion material travel. The channel causes the extrusion material 102 and the substrate 104 to converge such that the extrusion material 102 coats a surface of the substrate 104. An output portion of the channel generally defines a land length that operates to set the final shape of the extrusion material coating on the substrate 104 and complete the coating process.
For some applications, such as forming lumens in tubing, the system 100 includes a supply of pressurized gas 114 such as air, nitrogen, or other suitable gas. The pressurized gas 114 is fed through lumen channels in the extrusion die at a desired volumetric flow rate along with the substrate 104 and the extrusion material 102. The flow of gas is injected into the flow of extrusion material and creates a hollow tube within the coating on the substrate.
After the substrate 104 has been coated with the extrusion material, the coated product is fed through the cooling bath 108, which cools the molten extrusion material. For hollow tube products, the substrate 104 is separated from the coating.
When developing an extrusion die to form a new product, the die is generally tooled to certain specifications. With the configuration of the extrusion die channel generally fixed along the longitudinal axis in which the substrate and the extrusion material flow, the quality of the resultant product is highly dependent on the properties of the extrusion material. This is primarily due to the fact that the fixed channels of extrusion dies are generally configured to handle extrusion material having a particular rheology or flow characteristics. The flow characteristics of the extrusion material are dependent on its temperature, the pressure (generally fixed), and the material properties of the extrusion material (e.g., molecular weight). Accordingly, errors that are encountered during the development of an extrusion die must be corrected by either re-tooling the extrusion die, or by adjusting the settings of the flow of the extrusion material, such that the desired product can be produced.
Additionally, the tighter the tolerances of the product to be formed by the extrusion die and the more complex the product's structure is (e.g., one with lumens), the more difficult it is to develop the extrusion die due to the sensitivity of the resultant product on the rheology of the material flow. Unfortunately, different batches of the same type of extrusion material may have different molecular weights that can produce enough of a change in the rheology of the material to force an adjustment to either the extrusion die, or the temperature or pressure of the material flow, all of which can be very time-consuming.
There is a continuing need for improvements to extrusion dies including improvements that allow for more efficient development of extrusion dies for forming a new product.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.